United States Patent 3,700,433 ENHANCEMENT 0F TRANSVERSE PROPERTIES OF DIRECTIONALLY SOLIDIFIED SUPERALLOYS David N. Duhl, Newington, Couu., assignor to United Aircraft Corporation, East Hartford, Conn. No Drawing. Filed July 12, 1971, Ser. No. 161,912 Int. Cl. C22c 19/00 US. Cl. 148-325 4 Claims ABSTRACT OF THE DISCLOSURE The mechanical properties of the columnar-grained, nickel-base superalloys in the transverse direction are significantly improved by the addition thereto of about 1 percent zirconium.
BACKGROUND OF THE INVENTION The present invention relates in general to the nickelbase superalloys particularly as unidirectionally solidified into columnar-grained castings.
In the patent to VerSnyder 3,260,505, of common assignee with the present invention, reference is made to the production of castings, such as turbine blades, by unidirectional solidification techniques. Castings so pro duced have an elongated, columnar macro-grain structure with substantially unidirectional crystals aligned therein; that is, with a columnar structure in the castings. As a turbine blade, the grain boundaries are oriented to be substantially parallel to the principal stress axis of the blades, and there is an almost complete elimination of grain boundaries normal to this stress axis.
The mechanical properties of the columnar grain casting, including both strength and ductility, parallel to the columnar grains are superior to those obtainable by conventional casting. The mechanical properties in the transverse direction, however, viz perpendicular to the columnar grains, are not as high as those in the longitudinal direction. With current developments in cooled turbine hardware and the related thermal stresses incident thereto strength improvements in the transverse properties of directionally-solidified castings are very desirable.
A particularly desirable alloy for turbine blade applications is that referred to in the VerSnyder patent as SM- 200 (now known as MAR-M200), having a nominal chemical composition of, by weight, 9 percent chromium, 10 percent cobalt, 12.5 percent tungsten, 1 percent columbium, percent aluminum, 2 percent titanium, 1.5 percent iron, 0.015 percent boron, 0.05 percent zirconium, 0.15 percent carbon, balance essentially nickel. In the patent to Gell et al. 3,567,526, the substantial elimination of carbon is recommended.
As hereinafter discussed in greater detail, the present invention contemplates the inclusion of about 1 percent zirconium in nickel-base superalloy castings characterized by a columnar grained microstructure. It is known in the art that the hot workability of alloys may sometimes be improved by the addition of zirconium, cerium, hafnium or yttrium, as indicated in an article entitled A New Approach to the Problem of the Workability of Nickel- Chromium Alloys, Parry et al., Journal of the Institute of Metals (1969), vol. 97. Furthermore, certain prior art compositions, such as those referred to in the patent to Freche et al., 3,276,866, frequently describe zirconium contents of up to two percent when referring to alloys "ice having sufiicient ductility to facilitate fabrication of the alloy into sheet. In describing the alloy, designated WAZ- 20, the above inventors in the report NASA TN D-5352 available from the Clearinghouse for Federal Scientific and Technical Information observe that zirconium and trace amounts of other elements were picked up from the crucible during induction melting.
SUMMARY OF THE INVENTION The present invention contemplates the inclusion of about 1 weight percent zirconium to the nickel-base superalloy castings characterized by a columnar grained microstructure resultant from controlled unidirectional solidification techniques, to provide improved mechanical properties transverse to the direction of columnar grain alignment.
A maximum zirconium content of about 1.25 weight percent is required with the advantageous results being obtained within the zirconium content range of about 0.75-1.25 percent.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The nickel-base superalloys are recognized as those having high strengths at high temperatures such as those associated with the hot section of gas turbine engines. It is from this class of alloys that blades and vanes for use in gas turbine engines are preferably formed. As a class, these alloys are usually characterized by the inclusion of aluminum and/or titanium to promote the formation of the Ni (Ti,Al) intermetallic (7' phase), strengthening a nickel-chromium solid solution matrix. These alloys also normally contain elements like cobalt, and the refractory metals such as tungsten, molybdenum, etc. Typical of such superalloys are those referred to in the VerSnyder patent.
The addiiton of zirconium to the directionally solidified alloy casting will significantly improve its properties in a direction transverse to the growth direction. Illustrative of this effect is the enhancement of transverse properties to the MAR-M200 alloy resultant from the addition of 1 percent zirconium as shown in Table I. At 1400" F. (ductility minimum in this alloy) the increase in creep is most striking while at 1800 F. the creep elongation is enhanced. The significant increase in 1400 F. properties is most desirable in gas turbine hardware. It will also be noted that at a zirconium content of 1.3 weight percent, the transverse properties are inferior even to the basic MAR-M200 composition which for the purposes of the TABLE I Weight percent Zirconium (PWA 664) 0.8 1.15 1.3
Test F/K s.i. Life Percent E Life Percent E Life Percent E Lite Percent E A. Transverse:
1,400/100 i 1 2.8 224.5 5. 7 B.O.L. 1,800/25. 49 1. 3 2. 3 1,800] 40 1.3 5. 1 B. Longitudinal:
l B.O.L.=broke on loading.
With the zirconium addition, the transverse properties of the directionally-solidified columnar-grained castings have been brought to strength levels in the transverse plane approaching the strengths in the longitudinal direction, thereby reducing the extent of anisotropy in ductility and strength therein. Furthermore, consonant with the transverse property improvement, the desirable longitudinal proper-ties of the columnar-grained material (enhanced thermal fatigue resistance, enhanced longitudinal creep properties and decreased longitudinal modulus) are still retained.
The invention in its broader aspects is not limited to the specific steps, process and compositions shown and described but departures may be made therefrom within the scope of the appended claims without departure from the principles of the invention and without sacrificing its chief advantages.
What is claimed is:
1. In the production of a columnar-grained cast article by the unidirectional solidification of a nickel-base superalloy, the method of improving the mechanical properties transverse to the alignment of the columnar grains which comprises incorporating 0.75-1.25 weight percent zirconium in the superalloy composition.
2. As an article of manufacture, a cast article comprisa nickel-base superalloy containing about 0.75-1.25
weight percent zirconium characterized by a columnar grained microstructure with the columnar grains substantially aligned parallel to the principal stress axis of the article and characterized further by strength and creep properties transverse to said stress axis approaching those parallel thereto.
3. As an article of manufacture, a cast article comprising:
a superalloy which consists essentially of, by weight, about 9 percent chromium, 10 percent cobalt, 12.5 percent tungsten, 1 percent columbium, 2 percent titanium, 5 percent aluminum, 1 percent zirconium, up to about 0.20 percent carbon, 0.02 percent boron, balance substantially nickel, characterized by a columnar grained microstructure with the columnar grains substantially aligned parallel to the principal stress axis of the article and characterized further by strength and creep properties transverse to said stress axis approaching those parallel thereto.
4. A cast article according to claim 3 wherein:
the cast article is a gas contacting blade.
References Cited UNITED STATES PATENTS 3,260,505 7/1966 VerSnyder -171 RICHARD O. DEAN, Primary Examiner US. Cl. X.R.